When they were painting using egg yolks?

You're probably thinking of tempera paint, which was a popular painting technique used for centuries, especially in the Middle Ages and Renaissance.

Tempera paint is made by mixing pigments with an emulsion of egg yolks, water, and sometimes other binding agents like gum arabic.

Here's a little more information:

* History: Tempera painting has been used since ancient times, with notable examples found in ancient Egypt and Greece. It became very popular in the Middle Ages and Renaissance, with masters like Michelangelo, Raphael, and Leonardo da Vinci using it.

* Characteristics: Tempera paint dries quickly, creating a matte finish with vibrant colors. It is known for its durability and ability to withstand the test of time.

* Advantages: Tempera is a relatively inexpensive and easy-to-use medium. It is also well-suited for detailed work and fine lines.

* Disadvantages: Tempera paint can be brittle and prone to cracking if applied in thick layers. It is also sensitive to humidity and temperature changes.

So, while egg yolks are a key ingredient in tempera paint, it's not the only medium that uses them. There are also other egg-based painting techniques, like egg tempera, which uses only egg yolks as a binder, and oil tempera, which combines egg yolks with oil.

Let me know if you have any other questions about tempera paint or art history!